I run a number of 350 Chevrolet small blocks that range from bone stock to quite heavily modified.
If you have performed only normal modifications, and by that I mean you are running no more than 9.5:1 compression ratio, slightly warmed over cam, dual plane intake, headers, ignition upgrade, etc, a 600 will work fine.
You can run a 750 cfm *IF* you have vacuum secondaries on it, but you should run a fairly tall gear (numerically high) to keep the air intake velocity up, otherwise the larger primaries on the 750 will kill your low rpm torque.
If you calculate the cfm requirements, based on 80% volumetric efficiency, a 570 cfm carburetor is perfect for a 350 cubic inch engine. That makes the 600 just right in 95% of the cases. (Sorry, I can't remember the exact formula, but I remember the answer!)
Right now I have headers, HEI ignition, RV CAM, stock heads(but apparently they are the better of the stock ones), good exhaust, weiand action plus intake, 4.10 gearing. I should do well with a 600 then.